1. Technical Field of the Invention
The present invention relates generally to a pressure transducer such as a microphone designed to transform static pressure or dynamic pressure (e.g., acoustic vibration) into a corresponding electrical signal and a method of manufacturing the same.
2. Background Art
Japanese Patent Application No. 9-257618 teaches an electro-static capacitance type pressure sensor designed to convert the static or dynamic pressure into corresponding electrical signals. FIG. 7(h) shows this pressure sensor. FIGS. 7(a) to 7(g) show a sequence of manufacturing processes.
First, the substrate 30 is made of a monocrystalline silicon material. Impurities are diffused into a major outer surface of the substrate 30 to form the fixed electrode 40, the fixed electrode lead 41, and the lower fixed electrode terminal 42. Next, the first insulating layer 50, as shown in FIG. 7(a), is formed over the major outer surface of the substrate 30. On the first insulating layer 50, the sacrificial layer 60, as shown in FIG. 7(b), which is to be removed in a later process is formed.
The first insulating diaphragm layer 70, as shown in FIG. 7(c), is formed over the sacrificial layer 60. The second conductive layer 80 is formed on the first insulating diaphragm layer 70. Preselected portions of the second conductive layer 80 are removed to form the moving electrode 81, the moving electrode lead 82, and the lower moving electrode terminal 83.
Subsequently, the second insulating diaphragm layer 90, as shown in FIG. 7(d), is formed. A plurality of holes 91 are formed which extend to the sacrificial layer 60 through peripheral portions of the first and second insulating diaphragm layers 70 and 90. The holes 91 are used as etchant inlets.
Etching liquid is injected through the holes 91 to etch the sacrificial layer 60 isotropically to remove it, as shown in FIG. 7(e), thereby forming the reference pressure chamber 96 between the first insulating layer 50 and the first insulating diaphragm layer 70. The moving electrode connecting hole 92 and the fixed electrode connecting hole 94 are formed. The moving electrode connecting hole 92 extends to the lower moving electrode terminal 83 through the second insulating diaphragm layer 90. The fixed electrode connecting hole 94 extends to the lower fixed electrode terminal 42 through the second insulating diaphragm layer 90, the first insulating diaphragm layer 70, and the first insulating layer 50.
A conductive layer is formed on the second insulating diaphragm layer 90, after which preselected portions of the conductive layer are removed to form, as shown in FIG. 7(f), the moving electrode output terminal 93 and the fixed electrode output terminal 95. The moving electrode output terminal 93 connects with the lower moving electrode terminal 83 through the moving electrode connecting hole 92. The fixed electrode output terminal 95 connects with the lower fixed electrode terminal 42 through the fixed electrode connecting hole 94.
A sealing layer is formed on the second insulating diaphragm layer 90 to seal the holes 91 and then removed, as shown in FIG. 7(g), leaving portions around the holes 91 as sealing caps 97.
In operation, when the pressure is applied, it will cause a diaphragm consisting of the first and second insulating diaphragm layers 70 and 90 to be deformed. Specifically, both the pressure in the reference pressure chamber 96 and the surrounding pressure act on the diaphragm in opposite directions, so that the diaphragm is deformed by an amount equivalent to a difference between those pressures. This will cause the capacitance of a capacitor consisting of the moving electrode 81 formed on the diaphragm and the fixed electrode 41 to change as a function of the deformation of the diaphragm. The difference between the pressure in the reference pressure chamber 96 and the surrounding pressure acting on the diaphragm is, thus, determined by measuring the value of the capacitance. The measurement of absolute pressure may be accomplished by decreasing the pressure in the reference pressure chamber 96 to a level much lower than a pressure measurable range of the pressure sensor.
The above conventional pressure sensor, however, has the following drawbacks. When the etching liquid used to etch the sacrificial layer 60 and the cleaning solvent therefor are dried, the surface tension of the liquid may cause damage to the diaphragm. The avoidance of this problem requires an additional process of replacing the etching liquid and the cleaning solvent with liquid whose surface tension is smaller before drying them or of drying the etching liquid and the cleaning solvent using a gas liquefied by pressurizing and cooling it.
The formation of the holes 91 for feeding the etching liquid may cause the diaphragm to change in mass and compromise the mechanical strength. In order to minimize this problem, the holes 91 may be formed in the periphery of the diaphragm, however, the drawback is encountered in that it takes much time to etch a central portion of the diaphragm distant from the holes 91.
In a case where many pressure sensors are formed on a single substrate and separated using a dicing saw in mass production, the water used in the dicing will penetrate into cavities of the substrate, which may cause the pressure sensors to be broken when dried.
It is therefore a principal object of the present invention to avoid the disadvantages of the prior art.
It is another object of the present invention to provide a pressure transducer having the structure which allows the pressure transducer to be formed easily without damage to component parts such as a diaphragm etc.
According to one aspect of the invention, there is provided a pressure transducer designed to transform an applied pressure into a corresponding electrical signal. The pressure transducer comprises: (a) a substrate having a first surface and a second surface opposed to the first surface; (b) a fixed electrode formed in the first surface of the substrate; (c) a diaphragm attached at a peripheral portion thereof to the first surface of the substrate so as to form a cavity between a central portion thereof and the fixed electrode, the diaphragm having a moving electrode opposed to the fixed electrode through the cavity and being deformed in response to an applied pressure to change a distance between the moving electrode and the fixed electrode as a function of the applied pressure; and (d) a hole formed in the substrate which extends from the second surface to the cavity.
In the preferred mode of the invention, holes are further formed in the substrate which extend from the second surface to the cavity and which are so arranged that adjacent two of all of the holes are disposed at a regular interval away from each other.
The diaphragm is corrugated. Specifically, the diaphragm has a plurality of waved portions formed coaxially.
A groove is formed in the first surface of the substrate within the cavity and which leads to the holes.
A diaphragm support member is disposed within the cavity in contact with an inner wall of the peripheral portion of the diaphragm.
The substrate may be made of a semiconductor substrate having integrated circuit elements which form a detector designed to measure a capacitance between the fixed and moving electrodes.
The diaphragm may be made of an inorganic material such as a compound of silicon and one of oxygen and nitrogen.
The diaphragm may have a wave formed on the peripheral portion thereof. The wave projects to the first surface of the substrate to increase adhesion of the diaphragm to the first surface of the substrate. The wave may be formed by forming a groove in the first surface of the substrate so that the peripheral portion of said diaphragm partially projects to the groove.
According to the second aspect of the invention, there is provided a method of manufacturing a pressure transducer which comprises the steps of: (a) preparing a substrate having a first surface and a second surface opposed to the first surface; (b) forming a fixed electrode in the first surface of the substrate; (c) forming a sacrificial layer over the fixed electrode; (d) forming a diaphragm layer made of an insulating material over the sacrificial layer; (e) forming a hole which extends from the second surface of the substrate to the sacrificial layer; and (f) injecting gasses into the hole to remove the sacrificial layer in dry etching to form a cavity so that the diaphragm layer is deformed in response to an applied pressure.
In the preferred mode of the invention, the step of forming at least one waved portion on the first surface of the substrate may further be provided.
The waved portion may alternatively be formed on a surface of the sacrificial layer.
The substrate is made of a semiconductor substrate having integrated circuit elements which form a detector designed to measure a capacitance between the fixed and moving electrodes.
The diaphragm is made of an inorganic material, and the sacrificial layer is made of an organic material.
The diaphragm may be made from a compound of silicon and one of oxygen and nitrogen.
The sacrificial layer may be made of polyimide.
The removal of the sacrificial layer is achieved in the dry etching using oxygen plasma.
The gas injecting step removes the sacrificial layer so as to leave a peripheral portion of the sacrificial layer.
According to the third aspect of the invention, there is provided a method of manufacturing a pressure transducer which comprises the steps of: (a) preparing a substrate having a first surface and a second surface opposed to the first surface; (b) forming a fixed electrode in the first surface of the substrate; (c) forming an insulating layer over the fixed electrode; (d) forming a sacrificial layer on the insulating layer; (e) forming a diaphragm layer made of a conductive material over the sacrificial layer; (f) forming a hole which extends from the second surface of the substrate to the sacrificial layer; and (g) injecting gasses into the hole to remove the sacrificial layer in dry etching to form a cavity so that the diaphragm layer is deformed in response to an applied pressure.
In the preferred mode of the invention, the step of forming at least one waved portion on the first surface of the substrate is further provided.
The waved portion may alternatively formed on a surface of the sacrificial layer.
The substrate is made of a semiconductor substrate having integrated circuit elements which form a detector designed to measure a capacitance between the fixed and moving electrodes.
The diaphragm is made of an inorganic material, and the sacrificial layer is made of an organic material.
The diaphragm may be made form a compound of silicon and one of oxygen and nitrogen.
The sacrificial layer is made of polyimide.
The removal of the sacrificial layer is achieved in the dry etching using oxygen plasma.
The gas injecting step removes the sacrificial layer so as to leave a peripheral portion of the sacrificial layer.
According to the fourth aspect of the invention, there is provided a method of manufacturing a plurality of pressure transducers using a signal substrate which comprises the steps of: (a) preparing a single substrate having a first surface and a second surface opposed to the first surface; (b) forming fixed electrodes in the first surface of the substrate; (c) forming a sacrificial layer on each of the fixed electrode; (d) forming a diaphragm layer made of an insulating material over each of the sacrificial layer; (e) forming a hole which extends from the second surface of the substrate to each of the sacrificial layer; (f) forming a cutting groove between adjacent two of the pressure transducers for separating the pressure transducers from each other; and (g) injecting gasses into the hole to remove the sacrificial layer in dry etching to form a cavity so that the diaphragm layer is deformed in response to an applied pressure.